There are many industrial situations in which metal coils, web and other metal products or components impact on one or more wear surfaces. The wear surfaces are exposed to impact and shear forces exerted by the products or components with which they come into contact and the more durable the surface, the longer its life and the greater its economy. In heavy industrial applications wear surfaces are typically made from metal because of its strength. Most metals, however, have relatively high coefficients of friction, so softer metals with lower coefficients of friction, such as bronze, are often used so that the products or components can slide freely over the surfaces. An example of where metal wear surfaces are used is metal coiling operations in steel and aluminum mills.
Coiled metal is produced in aluminum or steel mills by compressing the metal billets between compression rollers until the desired thickness is achieved. Afterwards, the finished sheet is coiled around a mandrel. During the process the mandrel must open and collapse, thereby increasing and decreasing its diameter, to uniformly coil the metal sheet. Internal components, normally brass wedges, within the mandrel slide between the metal walls of the mandrel causing it to open and slide out causing the mandrel to collapse. Because of the weight of the metal and the forces exerted during compression and coiling, the wedges inside the mandrel and the wear surfaces over which the metal coil slides must be impact and shear resistant to withstand the forces exerted on them. Additionally, they must have a low coefficient of friction to facilitate the sliding of one part over another.
Historically, the wear surfaces and wedges used in the applications mentioned above have been produced of brass or bronze because of their strength and relative lubricity. Despite their lubricity the brass or bronze components still require additional lubrication which is accomplished by applying grease to the wear surfaces and internal wedges. Grease applied to the internal wedges is often centrifuged away because of the high rotational speeds of the mandrels. Furthermore, water or rust inhibitor is sometimes sprayed onto the metal coil or mill components while the mill is operating, which washes away some of the grease on the wear surfaces. Operators, therefore, are constantly applying grease to the bronze components which takes time and increases the manufacturing costs.
The relative lubricity and low coefficient of friction of brass or bronze is due mainly to the softness of the metals but, because of their softness, brass and bronze components wear out relatively quickly. This creates considerable expense because of the price of new components and the downtime required to replace them.
An alternative material to brass and bronze in these applications is polymer composite comprised of a polymer resin reinforced with carbon (graphite), glass or quartz fibers. Most fiber-reinforced polymer molding compounds have fibers with a length of 1/4" to 1". These composites are stronger than brass or bronze and they wear and last longer than brass or bronze in the aforementioned applications. Furthermore, the polymer resins in the composite are self-lubricating and have low coefficients of friction. Therefore, surfaces comprised of these composites do not have to be greased; however, the material cost of these composites is much higher than brass or bronze. Therefore, despite its superior performance and longer life, the overall higher cost to a manufacturing operation of components made from these composites renders their use prohibitive.
In order to take advantage of the superior characteristics of the abovementioned polymer composites, and to make a cost effective part, attempts were made to bond the composites to a metal substrate in order to reduce the amount of expensive composite material in each component, thereby reducing the cost of the component. The metal substrate used in these components was usually steel and a coating of approximately 1/4" to 1/2" of polymer composite was applied over the steel. The resulting components still proved to be expensive and impractical, primarily because the polymer composites did not bond well to the metal. Three methods of attachment were tried, chemical bonding, adhesive bonding and mechanical attachment. All three failed when the components were placed in the production environment.
If the polymer composite was to be chemically bonded to the metal; all oils and pickling compounds first had to be thoroughly removed from the metal's surface which required that the metal be cleaned with a solvent and dried. Then the polymer composite, usually supplied in the form of a sheet molding compound (SMC), was molded or otherwise formed over a surface of the substrate. Even when the metal was thoroughly cleaned, however, the chemical bonds which form between polymers and metals are weak and cannot withstand the high impact and shear forces in metal coiling and rolling applications.
To enhance the strength of the composite-to-metal substrate bond, adhesives were applied between the metal substrate and the composite. These adhesives often consisted of a prepreg polyimide composite having a polyimide resin as the adhesive and being reinforced with continuous glass or carbon fibers where the fibers comprise approximately 70-80% of the composite by weight. At the high temperatures (400.degree. F.-600.degree. F.) to which the finished parts were exposed, the adhesives proved to be ineffective. Metals and composites have far different rates of thermal expansion, composites's being slight relative to metals and the adhesive was not strong enough to withstand the expansion forces. Therefore, both chemical bonds and adhesive bonds proved to be ineffective at these temperatures, especially when the component was exposed to the stress and impact forces of the coiling operation. Under such conditions, the cladding composite material would simply pull away from the metal substrate.
Attempts made to mechanically attach the polymer to the metal substrate also proved unsuccessful. Mechanical attachment usually involved machining the surface of the metal substrate to create physical bonding sites, normally in the form of grooves. The polymer composite was then molded over the substrate and during the molding process some of the composite material would flow into and fill the grooves to form mechanical anchors for the cladding. This method also proved to be ineffective, partially because of the relatively short length of the fibers contained in the composites. The anchor is only as strong as the material of which it is formed and the tensile strength of the fibers within a composite material is much greater than that of the polymer. If the fibers are not long enough to extend from the clad surface into the groove and mechanically interlock with the wall of the groove, only the polymer forms an anchor for the composite. Because the tensile strength of the polymer is far less than that of the reinforcing fiber, the anchor formed by the polymer alone is not very strong and the cladding will still pull away from the metal substrate during use.
Recently, a new family of polymer composites was developed, the HyComp 300.TM. series, having PMR-15 polyimide resin and 1/2" to 2" fibers, and which is disclosed in U.S. Pat. No. 5,126,085. This product is unique in that it is a polyimide sheet molding compound (SMC) containing 1/2" to 2" long fibers rather than 1/4" to 1" fibers or continuous fibers which had been used in the prior art composites. Furthermore, the polyimide resin has a flow viscosity of 10.sup.2 -10.sup.5 centipoise at its melting temperature of 520.degree. F. prior to cross-linking. This viscosity is high enough so that when the resin flows it carries the fibers with it and, the viscosity is low enough so the resin carrying the fibers can flow easily into relatively small grooves and can conform to intricately shaped substrates during the molding process.
After molding and cross-linking, HyComp 300.TM. has a number of advantages as compared with other polymer composites. The longer, 1/2" to 2", fibers greatly increased the tensile strength of the material as compared to standard polyimide composites using shorter fibers. Whereas the tensile strength of standard polyimide composites is 10,000-15,000 psi, the tensile strength of HyComp 300.TM. composite is approximately 50,000-60,000 psi. When cross-linked and formed into a finished product, HyComp 300.TM. composite exhibits low friction, high wear resistance, low creep and good dimensional stability.
Because of these characteristics, HyComp 300.TM. composite had a longer life than short-fiber composites in metal coiling and rolling applications. Despite its longer life, however, HyComp 300.TM. composite still had a relatively high material cost and proved to be more expensive overall than brass or bronze when an entire component was produced from the composite material. Applicants then sought to develop a product comprised of a substrate material clad with the new HyComp 300.TM. composite. Applicants have discovered that specifically-configured discontinuity(ies) formed into the surface of a substrate, coupled with the use of polymer composites having 1/2" to 2" long fibers, create a component in which the cladding is firmly anchored onto the substrate and which is suitable for use in high stress, high temperature applications.